1. Field of the Invention
The present invention relates to a stabilized DC (direct current) power supply device. More particularly, the present invention relates to a stabilized DC power supply device having an output transistor that converts a voltage inputted thereto to an output voltage and delivers the output voltage.
2. Description of the Prior Art
Among conventional stabilized DC power supply devices, there is a type of stabilized DC power supply device that uses an externally connected output transistor so that properties thereof can be changed in order to cope with a small-current output as well as a large-current output.
FIG. 3 is a circuit diagram showing a configuration example of a conventional stabilized DC power supply device having an externally connected output transistor. A positive side of a DC power source 2 is connected to a drain of an n-channel MOSFET 1 (metal-oxide semiconductor field-effect transistor) connected externally as an output transistor and a negative side of the DC power source 2 is connected to ground. A source of the MOSFET 1 is connected to an output terminal 3 by way of a current-sensing resistor R1. One side of a load resistor RL is connected to the output terminal 3 and another side thereof is connected to ground.
The DC power source puts out a voltage VIN. It is to be noted that a value of the voltage VIN changes in accordance with a power source to be used as the DC power source 2. For example, the value of the voltage VIN differs depending on whether the DC power source 2 is a battery or a DC adapter. The MOSFET 1 delivers from the source thereof a voltage whose value is lower than the voltage VIN by a voltage incurred as a result of a voltage drop between the source and the drain thereof. Here, the voltage between the source and the drain of the MOSFET 1 changes according to a control signal to be fed to a gate thereof. Accordingly, an output voltage Vo at the output terminal 3 is equal to a value obtained by subtracting the voltage between the source and drain of the MOSFET 1 from the voltage VIN. Note that a voltage drop incurred by the current-sensing resistor R1 is so small that it can be ignored in this description. The output voltage Vo becomes equal to a reference voltage VREF due to a negative feedback of an operational amplifier 4 and is fed out from the output terminal 3.
At the same time, an inverting input terminal of the operational amplifier 4 is connected to a node between the current-sensing resistor R1 and the output terminal 3. A positive side of a reference voltage source 5 is connected to a non-inverting input terminal of the operational amplifier 4 and a negative side of the reference voltage source 5 is connected to ground. Furthermore, an output terminal of the operational amplifier 4 is connected to the gate of the MOSFET 1.
The reference voltage source 5 delivers the reference voltage VREF. The operational amplifier 4 feeds out the control signal that corresponds to a difference between the output voltage Vo and the reference voltage VREF. In this way, it is possible to maintain the output voltage Vo at a constant value even if the load RL changes or the value of voltage VIN is changed. The output voltage Vo is regulated at an identical level with the reference voltage VREF by way of a negative feedback operation of the operational amplifier 4.
However, the stabilized DC power supply device shown in FIG. 3 operates in such a way as to reduce the output voltage Vo by way of restricting a drain current of the MOSFET 1 when the drain current increases so that an output current Io flowing through the load resistor RL is prevented from causing an overcurrent situation. A protection circuit for restricting the drain current is composed of the current sensing resistor R1, an operational amplifier 6, an operational amplifier 7, a constant current source 8, and an external resistor R2.
A non-inverting input terminal of the operational amplifier 6 is connected to a node between the MOSFET 1 and the resistor R1, and an inverting input terminal of the operational amplifier 6 is connected to a node between the resistor R1, the output terminal 3, and the operational amplifier 4. Furthermore, an output terminal of the operational amplifier 6 is connected to a non-inverting input terminal of the operational amplifier 7.
Also, one side of the constant current source 8 and one side of the external resistor R2 are connected to an inverting input terminal of the operational amplifier 7. A constant voltage Vc is supplied to the constant current source 8. Another side of the external resistor R2 is connected to ground. A signal fed out from an output terminal of the operational amplifier 7 achieves a gain control of the operational amplifier 4.
The protection circuit configured in said manner will operate as described below. A source current of the MOSFET 1 flows through the current-sensing resistor R1. Then, the operational amplifier 6 detects a potential difference across the current-sensing resistor R1 and feeds out a voltage signal corresponding to the potential difference. The operational amplifier 7 feeds out to the operational amplifier 4 a control signal corresponding to a voltage difference between the output of the operational amplifier 6 and a voltage determined by a resistance of the external resistor R2. The operational amplifier 4 changes its gain in accordance with the control signal fed from the operational amplifier 7 so that the drain current of the MOSFET is kept under a predetermined value to prevent the output current Io from causing an overcurrent situation. Accordingly, an Io-Vo characteristic of the conventional stabilized DC power supply device shown in FIG. 3 will be represented by a curve similar to “a cliff overhanging the sea” as shown in FIG. 4.
It is to be noted that the conventional stabilized DC power supply device shown in FIG. 3 comprises a semiconductor integrated circuit which incorporates the operational amplifiers 4, the operational amplifiers 6, the operational amplifiers 7, and the constant current source 8. Additionally, to that semiconductor integrated circuit, the MOSFET 1, the current-sensing resistor R1, and the external resistor R2 are externally connected respectively.
The stabilized DC power supply device shown in FIG. 3 is capable of preventing the output current Io from causing an overcurrent situation as described earlier. However, because the restricted value of the output current Io is fixed even if the voltage VIN changes, when the voltage VIN becomes high and makes a source-drain voltage also high, it is possible that resultant heat causes the MOSFET 1 to break down.
For such a stabilized DC power supply device configured in such a way that the output transistor is incorporated in the semiconductor integrated circuit, it is possible to activate a thermal shutdown and thereby prevent the output transistor from breaking down by heat. However, it is not possible to measure the temperature in proximity to the output transistor used for the conventional stabilized DC power supply device shown in FIG. 3, because the output transistor is connected externally.
Note that a power unit disclosed by the Japanese Patent Application Laid-Open No. H8-123560 reduces the output voltage fluctuation of the power unit which is a power regulator, and stabilizes voltage to be fed to a load device. Therefore, the invention is not purposed for preventing an FET that forms the regulator from breaking down by heat.